Wasm reactors built with Rust v1.67+ use `__main_void` as a start function.

Apparently nobody uses this bizarre configuration, but Wasm reactors (`-Z wasi-exec-model=reactor`) broke again, and starting with Rust v1.67 they export `__main_void` (no args, `u32` return) instead of `main`.

Funnily enough, we tests reactors in Rust SDK repo, and we even load them in Envoy... but we don't have any e2e integration tests there that execute anything past it.